The present invention relates to a videotex terminal which can be connected to a telephone network for asking for and receiving pages of characters from a server center also connected to the network, and comprising:
a control keyboard, PA0 a display screen, and PA0 a modem with an input connected to the keyboard, an output connected to the screen and a bidirectional access connected to a telephone line of said network, said modem being normally provided for transmitting data from said center at a first speed and data towards said center at a second speed. PA0 means for detecting a call, disposed in said telephone line, PA0 means for connecting said telephone line to said modem access, in response to a detected call, PA0 alarm means, PA0 and means, connected to the output of said modem, for detecting a first graphic data transmission request, and for controlling, in the case of a first graphic data transmission request, said printing means and, in the absence of detection of a first graphic data transmission request, said alarm means. PA0 call detection means, disposed in said telephone line, PA0 means for connecting said telephone line to said modem access, in response to a detected call, PA0 means, connected to the output of said modem, for detecting a first graphic data transmission request and controlling, in the case of a first graphic data transmission request, said printing device so that it prints said first graphic data, PA0 alarm means, and PA0 means connected to the output of said modem for, in the absence of a first graphic data transmission request, detecting a request for transmission of pages of characters and controlling, in the case of a request for the transmission of pages of characters, means for receiving these pages of characters and for controlling said alarm means in the absence of a request for the transmission of pages of characters.
Such a terminal, well known in France under the name of "Minitel" makes it possible particularly for private persons or enterprises to have access to all sorts of information contained in data banks to which the server centers may be connected, or else access to a number of services able to be rendered by these server centers.
The information which the user of such a terminal receives on his screen is in the form of pages of characters, it being understood that by "character" is meant here not only the conventional alphanumeric characters but also a certain number of symbols for causing certain simple graphisms to appear on the screen. These symbols are limited in number and in fact are processed like alphanumeric characters, i.e. each of them is coded before transmission, for example in the form of a group of 7 binary elements or bits, this group of bits being decoded after transmission and restored to the screen in the form of the corresponding symbol. Considering the limited number of such symbols, it is obvious that the graphisms which may thus be restored are summary, as any user of a "Minitel" has realized. For this reason, these symbols are often termed "semi-graphic".
If the user of such a "Minitel" terminal desires to keep a copy of the information displayed on the screen, he may connect, on the socket provided on the terminal for the connection of peripheral type equipment, printer capable of recopying the screen. Naturally, the graphisms thus printed are strictly identical to those appearing on the screen, the printer being adapted, at the output of the modem, for converting groups of 7 bits into characters and semi-graphic symbols to be printed.
The data communication procedure provides for the modem being bidirectional, the modulation speed allowing simultaneous exchanges at a speed V.sub.1 in one direction and at a lower speed V.sub.2 in the other direction. The data from the server center, which is normally the most numerous, is thus transmitted at speed V.sub.1 which is equal to 1200 bauds, whereas the data transmitted by the "Minitel" terminal towards the server center, which is normally simple control data, is transmitted at the speed V.sub.2 which is equal to 75 bauds. In some cases, it is possible to "reverse" the modem, for using speed V.sub.2 at reception and speed V.sub.1 at transmission. The data transmitted is generally in groups of 7 bits of useful information, in synchronous mode, 10 bits in all being required per group of 7 useful bits. In fact, a group of 7 useful bits is preceded by a synchronization or start bit and followed by a parity bit and a stop bit.
Furthermore, so that private persons and enterprises can exchange veritable graphic information such as a manuscript document, an explanatory diagram, a constructional plan or any other fixed image, there exist telecopying apparatus or telecopiers.
In a telecopier used for transmission, the document comprising the image to be transmitted is analyzed finely so that each of its image elements, or pixels, is individually coded. After compression of the information resulting from such analysis, the corresponding data, which may be veritably termed "graphic data" is transmitted via a modem over the telephone network, to another telecopier used for reception. The latter is provided, at the output of its own modem, with a circuit for receiving such graphic data which in particular decompresses it and decodes it and controls a printer, for example a thermal printer, which restores the original image, with very good resolution, at the present time 8 points per millimeter, for example.
The telecopy communication procedure comprises, for each dispatch, protocol phases which enclose the transfer phase during which a flow of graphic data is transmitted from the transmitting telecopier to the receiving telecopier. During the protocol phases, simultaneous exchanges are provided for in both directions at the same speed of 300 bauds. During the first protocol procedure which precedes the transfer properly speaking, the communicating telecopiers exchange in particular data relative to their identifications and their respective possibilities. They also proceed with training which allows them to evaluate, considering the quality of the telephone line which connects them together, the maximum speed at which the transfer may take place, this speed being for example 2400, 4800, 7200 or 9600 bauds at the present time.
After the transfer phase which takes place, in synchronous mode, at the speed determined during training, a second protocol phase takes place which makes possible in particular the transmission of an acknowledgement and freeing of the line. The performances of modems used in telecopiers are therefore adapted for providing the preceding functions.
At the present time a private person or an enterprise which desires to keep a written copy of the information received via videotex and who desires to be able to receive from some of his correspondents documents of graphic quality must then be equipped both with a "Minitel" terminal having a screen recopying printer and a telecopier. Such an installation is of a relatively high cost price. In addition, if this user only has a single telephone line for receiving calls relative to telephone conversations and those relative to telecopies, he can only receive telecopies if a person is present near the telephone set at the moment when it rings, for manually connecting the telephone line to the telecopier if the call is relative to the transmission of a telecopy. Finally, if this user needs graphic data of a certain quality, so not capable of being restored by the "Minitel" screen and by the screen recopying printer and which is held in a data bank, he must request, during a telephone conversation with an operator of this data bank or possibly during a "Minitel" exchange, for this data bank to call him back to send him such graphic data by telecopy. In both cases, he must interrupt his communication, free the line and wait for the call from the data bank for connecting his telecopier. Such a procedure is particularly inconvenient, both for the user and the data bank.